Update slider.cs

fix minor error

com.unity.uiwidgets/Runtime/cupertino/slider.cs

                         visualPosition = _value;
                         break;
                 }
-                return MathUtils.lerpFloat(_trackLeft + CupertinoThumbPainter.radius, _trackRight - CupertinoThumbPainter.radius, visualPosition);
+                return Mathf.Lerp(_trackLeft + CupertinoThumbPainter.radius, _trackRight - CupertinoThumbPainter.radius, visualPosition);
             }
         }